The present invention relates to a technology for decreasing losses in a switching power supply device having a transformer for voltage conversion. More specifically, the present invention relates to a technology effectively used for DC-DC converters (power supply devices of direct current to direct current power converter type) having a synchronous rectifier circuit at the transformer's secondary side.
Conventionally, there is known a DC-DC converter as shown in FIG. 17. Such DC-DC converter uses a full-bridge switching circuit for the primary circuit and a synchronous rectifier circuit for the secondary rectifier circuit.
The DC-DC converter in FIG. 17 is a switching power circuit having the following configuration. The full-bridge switching circuit comprises switches SA through SD and drives a primary coil of a transformer TS1 with alternate currents. Synchronous rectification switches SE and SF are used to rectify alternate voltages induced at the secondary coil. Choke coils L1 and L2 alternately store energy to generate a desired direct current voltage in proportion to a windings ratio between the primary and secondary coils. The switches SA through SF comprise MOSFETs, for example. Diodes and parasitic capacitors parallel connected to the switches SA through SD indicate body diodes and parasitic capacitors each of which is provided between the source and the drain of each MOSFET.
While diodes are conventionally used as rectifier elements at the secondary side, the switches SE and SF comprising MOSFETs are used here to decrease power losses due to a forward voltage of the diode. If a voltage drop occurs between the source and the drain due to on-resistance of the MOSFET, the voltage drop can be made smaller than a forward voltage of the diode.